Ok, here's the thing. There's a difference between detaching and enabling.
It's obvious that you are obsessing over this visit and thus any "positive" actions you take would probably lean towards enabling. The real trick is to realize, and I mean REALLY realize that you are not in control of this sitch or your W. If she wants to see him, she'll see him. If she wants to "do" him, likely she will no matter what you think about it, do about it, or say about it. I don't mean to be harsh but it's the hardest thing for us to do in these sitches; that is to let go of the percieved control.
You are only so caught up because you feel that whatever decision YOU make will affect the final outcome. The second hardest thing to do in your sitch, and it goes hand-in-hand with the first is to understand that this is not about you, what you are or are not doing, or what you may say next. It's just not. It WAS about you when things were going wrong, just like it was about her but now she is doing something that she THINKS is for her and her alone.
I know it hurts like hell to think she just doesn't care anymore and the truth is she DOES care but not in a way that is easily manipulated by you. She will have to find her way back to THAT kind of caring on her own.
SO, in the meantime, should you choose this DB path, you have to find a way to accept your role in this, which is simply to improve YOUR life and YOURSELF to the point where if a "new" marriage, one that will have to rise from the ashes of this affair, were to be possible, you would be ready to be the different, BETTER man you want to be.
On the flip side, REALLY letting go means firmly understanding that you HAVE to do for yourself right now because she may or may not be there to do for anymore.
This CAN be done but it's really hard. You are being asked to turn inward when all you can find inside you is pain. Do it anyway, learn to manage your pain and then slowly learn how to manage your OWN happiness.
Out of time for now. I will keep up with you.
As for the meeting, you don't do ANYTHING you don't want to do. Period. Your decisions need to reflect YOU wishes, not hers.